The present invention relates to a method for cleaning unbrazable metal parts sufficiently to render them brazable and more particularly, to a method for treating damaged gamma prime hardened alloy parts so that the parts can be repaired by brazing.
Late model gas turbine engines (for example, those on the Boeing 747, the DC-10, and the Lockheed 1011) employ in their turbine sections nickel based alloys that are gamma prime hardened. Many other engines contain such materials, and the use of gamma prime hardened alloys will no doubt increase in the future due to the desirable properties of these super alloys.
The problem is that engine parts made of such alloys are very expensive and, at present, are not repairable when crack damaged due to metal fatigue. Attempts to weld repair such components results in post-weld cracking. Likewise, conventional brazing cannot be affected because nickel-base braze alloys will not run on the gamma prime hardened surfaces. Thus, although a molten brazing alloy under high vacuum might stick new gamma prime hardened parts together, it has not previously been possible to place brazing alloy inside cracks in damaged gamma prime hardened alloy parts.
A successful braze is manifest when braze material is placed at the source of a crack (say 0.001 inch wide and one-half inch long) and, at brazing temperature, not only melts and sticks to the parent material, but also runs into and fills the length of the crack. Apparently, in use a gamma prime hardened alloy becomes oxidized (and/or sulfuridized) to the extent that the aluminum, titanium and chromium oxides (or sulfides) which coat the surface of the part, including the surfaces of the crack, prevent successful repair by brazing.
Accordingly, it has been recognized that such parts must be cleaned if they are to be brazed. One suggestion is to use chromium fluoride (CrF.sub.3) and hydrogen (H.sub.2) to clean damaged parts of gamma prime hardened alloys prior to a braze repair. It is speculated that the following reaction mechanism takes place: EQU (CrF.sub.3.3/2 H.sub.2 O)+H.sub.2 .DELTA. 3HF+Cr+H.sub.2 O+H.sub.2 ( 1) EQU MO.sub.x +HF.fwdarw.MF.sub.x +H.sub.2 O+M (2)
If, then , MF.sub.x is volatile at the reaction temperature, the oxide is effectively reduced and the base metal (M) should be brazable. However, uniform reproducibility of results is for some reason lacking and many parts cleaned by this process are still not brazable.
It is also known that stainless steel can be brazed in a stable reducing atmosphere of fluoride. In a paper presented by the Toulouse, France, Microturbo Company representatives at the American Welding Society (AWS) meeting in Philadelphia in April 1977, entitled "Brazing Stainless Steel in a Stable Reducing Atmosphere of Fluoride," there is described a brazing process carried out in a halogen atmosphere obtained by the decomposition of fluorine salts such as ammonium bifluoride acid and chromium fluoride. The proposed reactions are:
(1) NH.sub.4 F.HF+Cr .sub..DELTA. CrF.sub.3 +NH.sub.3 .uparw.+H.sub.2 .uparw. PA0 (2) NH.sub.3 .fwdarw.1/2N.sub.2 +3/2H.sub.2 (on contact with metal) PA0 (3) CrF.sub.3 +H.sub.2 .uparw..fwdarw.2HF.uparw.+Cr PA0 (4) 6HF+Cr.sub.2 O.sub.3 .fwdarw.2CrF.sub.3 +H.sub.2 O.uparw.+F.sub.2 .uparw.
There are two pertinent observations regarding these reactions: (a) the object would appear to be the production of HF gas which, in turn, does the cleaning, and (b) any elemental fluorine that forms is produced downstream of the work piece (see reaction 4). It is noted that in the presented paper there is an indication that "the technique cannot be used on assemblies of materials having a high level of electroposivity, such as titanium and ziroconium," and "it is essential to avoid the introduction of carbon into the furnace during brazing." There is no suggestion in the presented paper that this process could be used to braze gamma prime hardened alloys, let alone clean damaged parts of such alloys prior to brazing.
Accordingly, the need exists for an effective method for cleaning damaged parts of gamma prime hardened alloys in order to render them braze repairable.